This invention relates generally to inductors and more specifically to a magnetically biased inductor, which is well suited for use as a filter choke in a motor drive system.
In motor drive systems, an inductor, usually taking the form of a filter choke, is coupled between the power supply and the inverter to filter the incoming inverter current. For the filter choke inductor to function effectively, the filter choke inductor must be properly sized, that is to say, the inductance and current rating of the filter choke inductor must be selected so that the filter choke inductor does not become magnetically saturated under normal current loads. At saturation, very little additional magnetic flux may permeate the magnetic circuit of the inductor, thus decreasing the incremental inductor inductance which lessens the ability of the inductor to filter ripple. Therefore, it is desirable to size the filter choke inductor so that at desired current amplitude, the inductor remains unsaturated.
To increase the current rating of an iron core filter choke inductor, a common practice is to provide an air gap within the inductor magnetic circuit. The presence of an air gap serves to decrease the overall magnetic permeability which in turn serves to decrease the inductance while increasing the current rating of the inductor. When an air gap is present in the magnetic circuit, the overall inductor reluctance is increased so that more magneto motive force (M.M.F.) is required to saturate the inductor as compared to the amount of M.M.F. required to saturate a closed path, that is to say a non air gap iron core inductor. Since the magnetic flux created in the inductor magnetic circuit is directly related to the current in the inductor, increasing the amount of M.M.F. required to saturate the inductor translates directly into an increased inductor current rating. However, the air gap reduces the overall magnetic permeability of the inductor, thereby decreasing the incremental inductance of the inductor. Thus, to obtain the same rated inductance, the air gap filter choke inductor must have a larger magnetic circuit in comparison with a closed path iron core filter choke inductor thereby increasing the overall inductor construction cost.
In an effort to obtain the advantages of an air gap inductor, namely an increased current rating without its associated disadvantage of decreased inductance, magnetically biased inductors have been developed. In the past the magnetically biased inductor has been fabricated by placing a rare earth magnet within the air gap such that the rare earth magnet M.M.F. opposes the M.M.F. of the inductor magnetic circuit. The presence of a rare earth magnet within the air gap of the inductor magnetic surface effectively serves to reverse bias the inductor magnetic circuit so that a greater amount of M.M.F. is required to saturate the inductor magnetic circuit as compared to the same inductor having an unfilled air gap. Yet, the presence of the rare earth magnet does not significantly change the overall magnetic permeability of the inductor magnetic circuit so that the inductor inductance is not diminished. Thus, by adding a rare earth magnet within the air gap of the inductor magnetic circuit, the current rating of the inductor is effectively increased, without adversely affecting the inductor inductance.
Magnetically biased inductors employing rare earth magnets are relatively expensive, due to the high cost of rare earth permanent magnet material. Moreover, although magnetically biased inductors radiate less electromagnetic interference than do conventional air gap inductors, the amount of electromagnetic interference radiated by a rare earth-magnetically biased inductor may still adversely affect the operation of other circuit components.